The present invention relates to classifying apparatus and method and, more particularly, to a vibrating screening deck and method of classifying and separating solid particulate materials of larger and smaller sizes from each other.
Vibrating screening decks have been widely used in the past in the classification and separation of particulate solids of varying particle sizes and compositions, such as limestone, coal and ores. Such screening decks have typically comprised a generally rectangular frame which is suspended in operation and which has a screen cloth in the frame upon which the materials to be classified are deposited. The frame and screen in such decks are suspended at an angle inclined from the horizontal, and at least the screen is vibrated to cause the solid particulates to move down the screen. As the materials move down the vibrating screen solids of smaller mesh size pass through the screen as "unders", and solids of larger particle size are discharged from the lower end of the screen as "overs".
One advanced form of such prior screening deck is disclosed in U.S. Pat. No. 4,444,656. That screening deck comprises a rigid frame in which the screens of the deck are mounted in the frame on resilient mountings. The screens are vibrated directly and independently of the frame by vibrating tappets which bear against the underside of the screens to impart high frequency, low amplitude vibrations directly to the screen. The advantages of the apparatus and method disclosed in the patent are that a substantial reduction in power consumption, apparatus weight and frequency of maintenance are realized over the screening apparatus and methods known previously.
Screening decks and methods as disclosed in U.S. Pat. No. 4,444,656 function quite well in the separation of particulate solids where the "unders" are larger than about 18 mesh. Where the "unders" exceed a size of 18 mesh, the screening deck is typically inclined at approximately 38.degree..+-.5.degree.. This is the angle of repose of most materials and at this angle the particulate materials slide down the vibrating screens by gravity. The .+-.5.degree. is adjusted to control the bed depth of the solid particulates on the screen which in turn controls to a degree the gradation of the materials that is to be separated.
At the foregoing 38.degree. angle of incline, screens are typically employed in which the mesh is formed with slotted rectangular openings to compensate for the relatively steep angle of incline of the screens. If a mesh having square openings was used, the gradation of the material would be effected to produce more fines because the particulate solids sliding across the surface of the screen do not actually meet the opening perpendicular to the screen at such incline. However, when screening "unders" below an 18 mesh opening, the rectangular slotted weave is no longer practical because wire cloth manufacturers either cannot weave such screens or, if they can, the cost becomes prohibitively high in such slotted configurations at such small mesh sizes. The present invention was developed to overcome the foregoing disadvantages.
In the present invention much finer gradations of smaller sized particles of "unders" are possible, and a square opening mesh which is readily and relatively inexpensively available in sizes as small as or smaller than 40 mesh may be utilized in place of the rectangular opening screens. Moreover, the likelihood of pegging or plugging of the screens having these small mesh sizes is substantially reduced by the apparatus and method of the present invention as will be described in more detail to follow.
In one principal aspect of the present invention classifying apparatus for separating particulate solids of larger and smaller sizes from each other includes a frame, at least one screen having openings therein to permit the passage of the smaller particulate solids therethrough, and mounting means for mounting the screen in the frame. First vibration means imparts high frequency, low amplitude vibrations to the screen and second vibration means imparts low frequency, high amplitude vibrations to the screen simultaneously with the high frequency, low amplitude vibrations.
In another principal aspect of the present invention a method of separating particulate solids of smaller and larger sizes from each other comprises introducing the solids to be separated to the inlet end of an inclined screen having openings therethrough and which is mounted in a frame, imparting high frequency, low amplitude vibrations to the screen while simultaneously imparting low frequency, high amplitude vibrations to the screen, and removing the smaller size particulate solids which pass through the vibrating screen and the larger size particulate solids which do not pass through the vibrating screen.
In still another principal aspect of the present invention, the foregoing first vibration means imparts the high frequency, low amplitude vibrations directly to the screen to vibrate the screen independently of the frame.
In still another principal aspect of the present invention, mounting means mounts the second vibration means to the frame to impart the low frequency, high amplitude vibrations to the frame and the screen.
In still another principal aspect of the present invention, the particulate solids to be separated are sequentially passed across a pair of screens, and each of the screens are vibrated with high frequency, low amplitude vibrations by the first vibration means independently of each other.
In still another principal aspect of the present invention, the high frequency, low amplitude vibrations have a frequency of between about 1,000-7,000 vpm and an amplitude of between 600-1,350 cfp and the low frequency, high amplitude vibrations have a frequency of between 900-3,600 vpm and an amplitude of between about 0-7850 cfp, and the frequency and amplitude of the high frequency, low amplitude vibrations are higher and lower respectively than the frequency and amplitude of the low frequency, high amplitude vibrations.
In still another principal aspect of the present invention, the aforementioned screen is inclined at an angle of about 20.degree. from the horizontal.
In still another principal aspect of the present invention, the openings in the screen are about 18 mesh or smaller in size, and the openings are approximately square in shape.
These and other objects, features and advantages of the present invention will be clearly understood through a consideration of the following detailed description.